The present invention relates to a multiterminal DC power transmission system in which AC power transmitted from an AC power system on the power transmission side is converted into corresponding DC power by a rectifier and the DC power transmitted is converted into corresponding AC power by using an inverter on the power receiving side.
A typical construction of the multiterminal DC power transmission system is a 4-terminal DC power transmission system as shown in FIG. 1. In the figure, AC power of AC systems 11 and 12 on the power transmitting side is coupled with rectifiers 15 and 16 through transformers 13 and 14. The AC power coupled is converted into corresponding DC power by the rectifiers 15 and 16 and supplied through circuit breakers 19 and 20 to ends of DC power transmission lines. The other ends of the DC transmission lines 17 and 18 are connected together at a connection point 23 through circuit breakers 21 and 22. The DC power transmission lines 17 and 18 further extend to the power receiving sides through corresponding circuit breakers 24, 25, 26 and 27 and coupled with inverters 28 and 29, respectively. The DC power transmitted is converted into corresponding AC power by the inverters and coupled with AC systems 32 and 33 through transformers 30 and 31, respectively. Incidentally, the rectifiers 15 and 16 on the power transmission side are each called as a forward converter and the inverters 28 and 29 on the power receiving side are each called an inverse power converter.
In order to stably operate the 4-terminal DC power transmission system, the DC power supplied through the rectifiers 15 and 16 must harmonically be supplied to the power receiving side through the inverters 28 and 29, respectively. This harmonical supply is also necessary not only for a case that the rectifiers 15 and 16 and the inverters 28 and 29 operate in a normal condition and also for a case that a fault such as ground fault F1 occurs on the DC transmission line 17 connected to the rectifier 15. In the case of the fault F1, only the DC power from the remaining rectifier 16 must well be parted and distributed to the inverters 28 and 29. To this end, the information representing operating conditions of the respective converters 15, 16, 28 and 29 must be collected through an information transmission system (not shown) to a central control unit and proper operating commands formed based on these items of the information must be applied to the converters 15, 16, 28 and 29. When a fault occurs in the power transmission system, the DC breaker quickly disconnects the fault location from the power transmission system. In this case, the fault information is supplied to the central control unit. Then, the central control unit issues commands to control the converters in order to keep a stable operation of the power transmission system against the fault. This necessitates expensive means for high speed information transmission and processing in the information transmission system and the central control unit. Therefore, desired is a control system which can obtain stable operating points on the DC voltages and currents in the converters, independently of the central processing unit and the information transmission system in an emergency.
In a known control system, all the converters have constant current control devices. The sum of the constant current control set values of the forward converters (rectifiers) are selected at a given value .DELTA.I (current margin) larger than the sum value of the constant current control values of the inverse converters (inverters). The converter with the lowest voltage determines the DC voltage in the power transmission system, while the remaining converters effect the constant current control in the power system.
FIG. 2 shows characteristic curves when the above-mentioned control system is applied for the power transmission system shown in FIG. 1. In the characteristic curves of FIGS. 2(a) to 2(d), the forward converter 15 determines the voltage in the power transmission system (FIG. 2(a)), while the remaining converters 16, 28 and 29 perform the constant current control (FIGS. 2(b), to 2(d). In this case, the operating points are P1 to P4, respectively. In FIGS. 2(e) to 2(h), the inverse converter 28 determines the voltage by a fixed marginal phase angle control (FIG. 2(g)), while the remaining converters perform the constant current control (FIGS. 2(e), 2(f) and 2(g)). In this case, the operating points are P'1 to P'4, respectively. Further, the voltage in the DC system is controlled by adjusting tap positions of the transformers. In a normal operating mode, a single converter determines the voltage in the power transmission system, while the remaining converters control the current, thereby obtaining a stable operation.
This control system, however, is not suitable for the case where the fault location is quickly disconnected from the power transmission system by using the DC breaker. Specifically, when the ground fault in the DC system or the fault converter is quickly removed, it is very difficult to obtain new stable points after the fault is removed.
Assumed that, in FIG. 1, when the ground fault F1 occurs in the DC system, and the fault F1 is removed quickly by the DC breakers 19 and 21. In this case, if the power transmission system operates as indicated by the characteristic curves as shown in FIGS. 2(a) to 2(d), the DC current values Id1 of the forward converter is zero. For this reason, a relationship between the current value Id1 of the forward converter 16 and the current values Id3 and Id4 of the inverse converters {Id2-(Id3+Id4)} is negative and the current margin .DELTA.I is not secured. Therefore, at a normal voltage, stable operating points can not be obtained and the direction of power flow may be inverted. In another case where the ground fault F4 takes place in the DC power transmission system in FIG. 1 and it is quickly removed by means of the DC breakers 25 and 27, if the power transmission system operates as indicated by the characteristic curves in FIGS. 2(e) to 2(h), the DC current Id4 of the inverse converter 29 becomes zero. Therefore, the sum of the output currents Id1 and Id2 of the forward converters 15 and 16 flows into the inverse converter 29, so that the sound converter 28 is forced to be in an overload condition.
For this reason, before removing the fault by the DC breaker, the information representing the fault and operating conditions of the respective converters must be transmitted to the central control unit where a power redistribution is calculated and the central control unit must give new set values of the DC currents to the converters. In this case, however, time consuming factors such as transmission delay, acknowledgements of signals greatly damage the merit of the high speed operable DC breakers provided. Furthermore, the reduction of the quantity of power transmission due to the continuity of the fault provides a great hindrance in stably supplying the power.
For an arm short-circuiting fault of the forward converter as well as the ground fault in the DC system, the converter must be gate-blocked at high speed to disconnect the converter from the DC system for obtaining a correct current distribution. For a fault in the transmission system, a set value of the constant current control unit associated with only the specific converter must be changed. If not done so, the direction of power flow is inverted or the overload problem of the inverse converter arises.
In another known control system, the converters are provided with current adjustors, respectively. When the rectifier 15 in FIG. 1 is disconnected from the power transmission system, the operating voltages of the inverters 28 and 29 are set at values lower than the set values. And the operating current of the inverter is reduced by using the current adjustor. FIG. 3 shows control characteristics of the power transmission system incorporating the control system. FIGS. 3(b) and 3(c) show current voltage characteristic curves of the inverters 28 and 29, respectively. In a stationary condition, the system voltage is set at VD1 and the inverters 28 and 29 receive the set currents Id3 and Id4 from the rectifiers 15 and 16, respectively. When the fault F1 takes place and the rectifier 15 is disconnected from the system, the operating voltages of the inverters 28 and 29 are reduced to Vd2 (&lt;VD1). As a result, the current of the inverters 28 and 29 reduces from the set values Id3 and Id4 along the curve of the voltage Vd2. Generally, the impedances of the inverters 28 and 29, as seen from the rectifier 16, are different from each other. For example, when the impedance of the inverter 28 is lower than that of the other inverter, the current from the rectifier 16 flowing into the inverter 28 is larger than the current into the other inverter. In an extreme case, the current Id4 flowing into the inverter 29 may be zero.